Cultivated soybean (Glycine max) has a substantial commercial value throughout the world. Over 50 million hectares worldwide are used to produce an annual crop of soybeans in excess of 100 metric tons with an estimated value exceeding 20 billion dollars. The development of scientific methods useful in improving the quantity and quality of this crop is, therefore, of significant commercial interest.
Modem biotechnological research and development has provided useful techniques for the improvement of agricultural products by plant genetic engineering. Plant genetic engineering involves the transfer of a desired gene or genes into the inheritable germline of crop plants such that those genes can be bred into or among the elite varieties used in modem agriculture. Gene transfer techniques allow the development of new classes of elite crop varieties with improved disease resistance, herbicide tolerance, and increased nutritional value. Various methods have been developed for transferring genes into plant tissues including high velocity microprojection, microinjection, electroporation, direct DNA uptake, and Agrobacterium-mediated gene transformation.
Agrobacterium-mediated gene transformation is the most widely used gene transfer technique in plants. This technique takes advantage of the pathogenicity of the soil dwelling bacteria, Agrobacterium tumefaciens or Agrobacterium rhizogenes. Agrobacterium tumefaciens natively has the ability to transfer a portion of its DNA, called T-DNA, into the genome of the cells of a plant to induce those cells to produce metabolites useful for the bacterium's nutrition. Agrobacterium-mediated transformation takes advantage of this concept by replacing the T-DNA of an Agrobacterium with a foreign set of genes, thus, making the bacterium a vector capable of transferring the foreign genes into the genome of the plant cell. Typically, the foreign gene construct that is transferred into the plant cell involves a specific gene of interest, which is desired to be introduced into the germline of the plant, coupled with a selectable marker that confers upon the plant cell a resistance to a chemical selection agent. Typically, the Agrobacterium-mediated gene transfer is into an undifferentiated cell cultivated in tissue culture, known as a callus cell, or the transfer is made into a differentiated plant cell from a leaf or stem, which is then induced to become an undifferentiated callus culture.
Although significant advances have been made in the field of Agrobacterium-mediated transformation methods, a need continues to exist for improved methods to facilitate the ease, speed and efficiency of such methods for transformation of soybean plants.